This invention relates to the preparation of metal surfaces for finishing operations, especially the application of siccative finishes. More particularly, this invention relates to a final rinse for conversion coated metal surfaces, comprising an aqueous solution of a selected organosilane and a selected organotitanate, which enhances the adhesion of siccative coatings, and imparts improved corrosion resistance to such surfaces.
The surfaces of metal articles are commonly coated with siccative finishes for a variety of reasons including protection of the metal from corrosion and for aesthetic purposes. However, it is well-known that many organic and inorganic siccative coatings do not adhere well to the surfaces of articles made from metals (for example, iron, steel, aluminum, and zinc and their alloys) in the raw state, thereby degrading the corrosion inhibiting and aesthetic benefits of such coatings. To alleviate this problem, such surfaces are routinely treated with a "conversion coating" prior to the application of a siccative finish. The conversion coating acts as an adherent base for a subsequent siccative coating and also, in cooperation with the siccative coating, acts to inhibit corrosion of the metal surface. Such conversion coatings are well-known in the art and thus will not be described in detail.
When used alone, conversion coatings often provide less than acceptable levels of siccative coating adhesion and corrosion resistance. These properties are often improved upon by the subsequent application of a "final rinse", also termed a "post-treatment". Of the final rinses used heretofore, those usually considered most effective in improving the siccative coating adhesion and corrosion resistance properties of metallic surfaces generally contain chromic acid or other chromium-containing compounds. It has become well-known, however, that hexavalent and trivalent chromium compounds are hazardous and toxic. Consequently, solutions containing such chromium compounds present difficult handling problems in use. Also, they are currently prohibited by law from being introduced into natural waters or public sewage systems, and are therefore difficult and expensive to dispose of in an environmentally acceptable manner. For these reasons, substantial efforts have been directed toward the development of non-toxic, or less toxic, chromium-free final rinse solutions. These efforts have resulted in a wide variety of well-known chromium-free final rinses, having either organic or inorganic compositions.
Organofunctional silanes, also known as organosilanes, are widely used in various industries in diverse applications. Compounds of this class are primarily used as corrosion inhibitors, coupling compounds, or as adhesion promoters to enhance the bonding of organic coatings, including polymeric coatings, to metal, glass, or polymeric surfaces. Organosilanes have been utilized in final rinse solutions for the post-treatment of conversion coatings. For example, Japanese Patent Application No. 59-93188 (Public Patent Disclosure Bulletin 59-219478 Dated Dec. 10, 1984) describes a post-treatment agent for metal surfaces comprising an aqueous solution in which the active ingredient is an organoalkoxysilane. Metal surfaces treated with such solutions are described as having improved corrosion resistance and siccative coating adhesion. According to this disclosure, the organoalkoxysilanes which may be employed include those whose reactive organic functional groups comprise a carbon-carbon double bond (e.g., vinyltrimethoxysilane and vinyltriethoxysilane), an epoxy group (e.g., 3-glycidoxypropyltrimethoxysilane), or an amino group (e.g., (2-aminoethyl) aminopropyltrimethoxysilane).
Polyfunctional organotitanates have a variety of well-known industrial uses including catalysis, polymer cross-linking, and surface modification. Such organotitanates have also been compounded with various silicon compounds, including organosilanes, for use on coating compositions. For example, U.S. Pat. No. 4,746,366, issued to G. Philipp et al. on May 24, 1988, describes a lacquer which has been obtained by the hydrolytic precondensation of: (a) at least one titanium or zirconium compound of the formula EQU MR.sub.4
in which M denotes titanium or zirconium and R represents halogen, hydroxyl, alkoxy, acyloxy, or a chelating ligand, preferably a ligand coordinated to the metal via oxygen and/or nitrogen; and (2) an organosilane. According to this patent, such lacquers, upon hardening by heating or exposure to ultraviolet radiation, produce scratch-resistant coatings suitable for use on a variety of substrates. Such coatings are described as providing particularly advantageous results when applied to scratch-sensitive plastics, including, for example, spectacle lenses.
U.S. Pat. No. 4,749,741, issued to M. Saito et al. on June 7, 1988, discloses a primer composition for cohesively bonding silicon rubbers to plastics and metals. This composition is comprised of: (a) an organotitanate ester; (b) optionally, either (1) an organosilicon compound or (2) an organohydrogensilane or an organohydrogenpolysiloxane; and (c) a quantity of organic liquid sufficient to solubilize the composition. The organotitanate ester has the general formula EQU [HC(R.sup.1).dbd.C(R.sup.2)CO(O)].sub.1 Ti[OC(O)R.sup.3 ].sub.m (OR.sup.4).sub.n
wherein R.sup.1 and R.sup.2 individually represent a hydrogen atom or an alkyl radical; R.sup.3 and R.sup.4 represent identical or different alkyl or aryl radicals; l =1, 2, or 3; m=0, 1, 2, or 3; n=0, 1, 2, or 3; and l+m+n=4. In the foregoing organotitanate ester, the titanium atom must be bonded to at least one residue resulting from the removal of a hydrogen atom from the carboxyl group of an ethylenically unsaturated monocarboxylic acid such as acrylic or methacrylic acid. The optional organosilicon compound has at least 2 silicon-bonded alkoxy groups in each molecule and has the average unit formula EQU R.sup.5.sub.u R.sup.6.sub.v (OR.sup.7).sub.w SiO.sub.( 4-u-v-w)/2
while the alternative optional component, which is either an organohydrogensilane or an organohydrogensiloxone, contains at least one silicon bonded hydrogen atom per molecule and has the average unit formula EQU H.sub.x R.sup.5.sub.y (OR.sup.7).sub.z SiO.sub.( 4-x-y-z)/2
where R.sup.5 represents a substituted or unsubstituted monovalent hydrocarbon radical exclusive of the substituents represented by R.sup.6 ; R.sup.6 represents a monovalent organic group containing an addition-reactive or condensation-reactive functional group; R.sup.7 represents an alkyl, aryl or alkoxyalkyl radical; the values of u and v are identical or different and are within the range of from 0 to 3, inclusive, the value of w is greater than 0 up to and including 4, the sum of u and v and w is greater than 0 up to and including 4, the value of x is from 1 up to but not including 3, the value of y is from 0 up to but not including 3, the value of z is from 0 to 3, inclusive, and the sum of x+y+z is greater than 0 up to and including 4.
A commercial product, comprising an organosilane and morpholine, or a morpholine-based derivative, is available for use as a final rinse for conversion coated metal surfaces.
The degree of siccative coating adhesion and corrosion resistance provided by such known compositions generally fails to achieve the highly desirable levels of those properties which are exhibited by chromium-based final rinses. Also, some of the known compositions are less than completely compatible with certain types of coatings or paints, resulting in undesirable blistering, cracking, peeling or chipping of the finish coat. Recently, I have discovered that an aqueous solution containing a selected organosilane and a selected organotitanate, when used as a final rinse on conversion coated metal surfaces, provides such surfaces with an exceptional degree of siccative finish adhesion and corrosion resistance, which is comparable to chromium-containing final rinses.